Where gas distribution systems employ compressors in parallel for compressing the gas and distributing the same through a discharge manifold and where a suction manifold extends from the end use device or devices and feeds the return gas to be compressed to the multiple, parallel compressors the gas is employed in the compressor for carrying oil for lubrication and other purposes. There is a tendency, depending upon the characteristics of the system distribution means and/or the compressors themselves for oil to excessively accumulate within one or more of the compressors, while others are simply starved of oil. Attempts have been made to provide bleed line connections between such compressors and utilize gas pressure differentials between the compressors to circulate oil to the starved compressor. Where the compressors constitute hermetic units, the bottom of the casing of the compressor acts as an oil sump for receiving accumulated oil and for supplying the oil to the moving parts of the compressor unit for both lubrication and sealing. Such oil distribution arrangement is the subject matter of applicant's earlier U.S. Pat. No. 3,237,852 as applied to a compressed gas distribution system employing multiple parallel hermetic motor compressor units of the reciprocating piston type.
Recently, there has been successful commercial exploitation of relatively small vertical axis helical screw rotary compressor hermetic units, wherein the helical screw compressor comprised of intermeshed rotary helical screws, being vertically oriented, are mounted for rotation about parallel, vertical axes within a hermetic compressor casing and with an electric drive motor for the helical screw rotors being vertically mounted above one of the helical screws, and with the rotor shaft for the electric motor constituting an extension of one of the helical screws. Further, in conjunction with an oil separation scheme, the interior of the hermetic casing is maintained at compressor discharge pressure with the major portion of the separated oil accumulating within the bottom of the casing and rising to a level less than the vertical height of the intermeshed helical screws and well below the level of the electric motor components. In the case of the helical screw rotary compressors, the oil is required not only for lubrication of the moving parts, but in addition performs an excellent seal between the intermeshed helical screws. Even though means are provided for extracting the oil from the compressed gas on the discharge side of the compressor such as centrifugal means as well as gas deflectors between the discharge port of the compressor below the electric drive motor and the casing discharge port, which is normally centrally located within the top wall or cover of the casing, some oil is carried away by the discharge gas. Due to the idiosyncrasies and individual characteristics of the machine and the distribution system, as mentioned previously, one or more of the parallel helical screw compressors will tend to accumulate an excess of oil, while the oil levels within the sump portion of the hermetic casing of another compressor will drop. This results in inadequate lubrication and sealing for those hermetic units having reduced oil supply, while the overaccumulation of oil in the unit or units tending to build up oil, may adversely affect proper operation of the helical screw compressor portion of those units.
It is therefore a primary object of the present invention to provide an improved oil equalization scheme for a rotary gas distribution system employing parallel, connected helical screw rotary compressor hermetic units in which overaccumulation of oil within the sump portion of the hermetic casing of any one of the units, automatically results in the bleed of accumulated oil from that unit for circulation within the gas distribution system and redistribution to the remaining compressor unit.
It is a further object of the present invention to provide an improved gas distribution system employing multiple, parallel helical screw rotary compressor hermetic units in which the compressor discharge pressure of a given unit is advantageously employed in effecting the redistribution of oil from the unit having an overaccumulation of oil to the other units in a simplified yet positive manner.